Process for improving the reflectivity of reflective surfaces of antennas

ABSTRACT

The present invention refers to a novel process of coating the reflective surfaces of antennas made of composite material. In particular, it refers to the reflective surfaces made of carbon fiber impregnated with epoxy resins of parabolic antennas. The present invention further refers to the surfaces and to the antennas thus obtained.

The present invention refers to a novel process of coating the reflective surfaces of antennas of composite material. In particular, it refers to reflective surfaces, made of carbon fibers impregnated with epoxy resins, of parabolic antennas. The present invention further refers to surfaces and to antennas thus obtained.

STATE OF THE PRIOR ART

In the telecommunications field, by “parabolic antenna” it is meant an antenna having an opening fitted with a parabolic mirror, and that can be used both in transmitting and receiving. An antenna is a device designed for the purpose of emitting or receiving electromagnetic waves towards open space. While a transmitter antenna transforms voltages into electromagnetic waves, a receiving antenna performs the reverse function. The traditional classification of antennas is essentially based on the manner in which the electromagnetic field is distributed in the antenna itself, or on the technology used, although classifications can also be made from a more practical point of view, based on their specific uses and their operation.

In the case of antennas with a reflector, the manner of producing and receiving electromagnetic waves is done through one or more reflective surfaces, also known as reflectors.

In many applications, instead of using metallic materials to construct the reflective surfaces, materials formed by a fiber (e.g., carbon fibers) and a resin matrix (e.g. epoxy resins) are used, which compared to metallic materials are lighter in weight and have better mechanical and corrosion strength.

These materials however have a reflectivity to electromagnetic waves which depends on their degree of surface finish expressed in roughness Ra (μm).

Therefore, as frequency values rise, reflectivity to the electromagnetic field degrades to unacceptable values. Hence, it is necessary to improve the reflectivity of these surfaces.

One of the processes reported in the state of the art for the improvement of the reflectivity of antenna reflectors of composite material (US20100288433) consists in their plating with thin metal laminations. This process entails drawbacks, such as the forming of microgaps in the bonding lines between a metallic strip and the contiguous one, and may require specific adhesives for binding these metallic strips to the carbon fiber substrate, adhesives that, under environmental conditions oft-times very difficult, have expansion modules completely different from the remainder of the antenna structure. Moreover, laminations give to the antenna an excessive weight, which actually nullifies the advantage of using lightweight materials such as carbon fibers.

Patent DE2061840B describes a method of coating reflective surfaces of antennas of a generic composite material; antennas thus obtained operate at low frequencies only.

U.S. Pat. No. 4,188,358 describes a method of coating reflective surfaces of parabolic antennas of fibrous material by thermal spray; antennas thus obtained only operate in the range comprised between 1 and 20 GHz.

Object of the present invention is to provide a novel process for improving the reflectivity of reflective surfaces of satellite antennas overcoming the above cited drawbacks, and to obtain antennas capable of receiving and transmitting efficiently even at 30 GHz.

SUMMARY OF THE INVENTION

The Inventor has set himself and solved the problem of increasing the reflectivity to microwaves of reflective surfaces of parabolic antennas of composite material so as to receive electromagnetic waves with high frequencies, up to 300 GHz.

It has surprisingly been found that by utilizing a determined process of metal deposition on composite materials (particularly prone to deformations even at temperatures exceeding 70-80° C.) and by selecting appropriate process conditions, it is possible to obtain a parabolic antenna that, though maintaining the advantages in terms of lightness of weight, mechanical strength, corrosion strength and ease of molding, peculiar to antennas of composite material, has a reflective surface with an average surface roughness and an adhesion between the metal coating and the underlying carbon fiber structure such as to be useful also with high-frequency electromagnetic waves.

In particular, as shown in FIGS. 1-4, the antennas without the metal coating deposited with the process of the present invention do not receive and transmit efficiently at 30 GHz frequencies (FIGS. 1 and 2), whereas the antennas on whose reflective surface a metal layer has been deposited according to the process of the present invention receive and transmit efficiently even at 30 GHz frequencies (FIGS. 3 and 4). Further tests (FIGS. 5 and 6) show that, without the step of abrasion of the surface layer of the epoxy matrix, the antennas after deposition of the metal layer are not capable of transmitting and receiving efficiently at 30 GHz. Therefore, object of the present invention are:

A process of coating the reflective surfaces of parabolic antennas, wherein said reflective surfaces are made of a composite material consisting in carbon fibers in epoxy resin matrix and wherein said epoxy resin matrix has a surface thickness comprised between 10 and 30 μm, comprising the following steps:

a) performing a step of abrasion by shot peening on said reflective surface so as to remove a layer of epoxy resin of at least 3 μm; b) performing a step of metal deposition on the reflective surface prepared at a) by the thermal wire spray technique, with one or more phases of cooling on the acceleration current of the metal particles and/or on the surface of the parabolic dish of said antenna.

Reflective surfaces of parabolic antennas with a metal coating obtainable according to the process of the present invention.

Parabolic antennas comprising the reflective surfaces obtainable according to the process of the present invention.

Still further advantages, as well as the features and the modes of employ of the present invention, will be made apparent in the following detailed description of some preferred embodiments thereof, given by way of example and not for limitative purposes. Reference will be made to the figures of the annexed drawings.

DETAILED DESCRIPTION OF THE FIGURES

FIGS. 1 and 2 show the 30 GHz-azimuth radiation pattern of parabolic antennas made of epoxy resin-reinforced carbon fiber. The antennas prove to be not suitable at 30 GHz (+/−30 GHz and +/−10 GHz scale) for angles comprised between −30 and 30 degrees (FIG. 1); and the detail between −10 and 10 degrees (FIG. 2).

FIGS. 3 and 4 show the 30 GHz-azimuth radiation pattern of the antennas of FIGS. 1 and 2 after the reflective surfaces were subjected to the process of the present invention according to the embodiment described in the examples. The antennas proved to be perfectly suitable at 30 GHz (+/−30 GHz and +/−10 GHz scale) for angles comprised between −30 and 30 degrees (FIG. 3); and the detail between −10 and 10 degrees (FIG. 4).

FIGS. 5 and 6 show the 30 GHz-azimuth radiation pattern of the antennas of FIGS. 1 and 2 after reflective surfaces were subjected to a metallization process without prior surface abrasion treatment according to the process of the present invention. The antennas proved to be not suitable at 30 GHz.

DETAILED DESCRIPTION OF THE INVENTION

According to a preferred embodiment, the process of the present invention comprises a step of metal deposition on a reflective surface of a parabolic antenna by a wire technique referred to as “thermal wire spray”.

Thermal wire spray generates molten or semi-molten metal microparticles in a plastic state, from a metal wire of Al, of Cu or other metal which is partially melt by the flame and then atomized in the form of microparticles, accelerated in the same nozzle of the system with an air current or an argon current until impacting on the surface to be coated.

The step of metal deposition by thermal wire spray is characterized by one or more phases of cooling. The phase of cooling could be obtained on the acceleration current of the metal particles and/or on the surface of the parabolic dish of the antenna; for instance, by blowing compressed air and/or compressed air mixed with crystals of solid CO₂ on the paraboloid surface not concerned by the coating, or in a refrigerated chamber (e.g., between −5 and 5° C.). With this system it is possible to keep low the surface temperature of the satellite dish constructed of a composite material (e.g., constructed of carbon fiber impregnated with epoxy resin). At the same time, a uniform coating of metal nano- and microparticles is obtained which, by impacting on the same surface without altering it, allows perfect maintaining of the parabolic geometric form, enabling optimum receiving/transmitting. In the step of deposition by thermal wire spray, oxide-acetylenic nozzles, or nozzles for oxygen mixed with combustible gases (such as, e.g. GPL, methane, propane) will preferably be used. Moreover, the acceleration fluid comprised of compressed air, nitrogen or argon, fluid which is outside the nozzle, is additioned with solid CO₂ to keep low the temperature of impact on the composite material of carbon fiber, and preventing even very tiny deformations thereof which might nullify the parabolic geometry of the antenna. Should a refrigerator circuit be used to cool the acceleration fluids of the metal particles, it is possible to do without the adding of CO₂ crystals.

The coating rate could be, e.g., comprised between 20 and 24 mm/sec, with a distance of about 5-20 cm from the surface to be coated. In the process of the present invention various metals or metal alloys could be used, like e.g. Al, Zn, steel (for instance of the AISI 304, 316 and 316L series) Cu, Ag, Au, Ni/AI and SbSn alloys.

To obtain a more uniform coating in the step of metal deposition, a rotational speed of the same dish could be activated, in the range comprised between 40-400 rev/min, preferably between 200 and 400 rev/min.

To improve the mechanical adhesiveness of the metal particles on the surface of carbon fiber dishes coming out of the molds, the process of the present invention will comprise a step preceding the metallization (metal deposition) step, in which a layer of the matrix of at least 3 micrometers (μm), preferably a layer comprised between 3 and 6 μm, is removed from the reflective surface of the parabolic antenna. For instance, in the case of satellite dishes of carbon fibers and epoxy resin, a layer of 3-4 μm could be removed from the surface layer of epoxy resin by utilizing a process for generating a light surface abrasion by a shot peening process. The surface layer of epoxy resin will be comprised between 10 and 30 μm, preferably between 15-20 μm. Abrasion could be performed by a shot peening process, carried out by glass microspheres capable of ensuring a surface roughness equal to 3-4 μm and a surface penetration of the metal nano- and microparticles suitable to create, on the same surface of the paraboloid, a perfect mechanical adhesiveness comprised between 4 and 10 MPa.

To further improve the surface of the metal coating, bringing it to a mean surface roughness (Ra) of 0.5-1 μm, the process could provide a (manual or automatic) final finishing process, to be carried out, for instance, with 3M abrasive papers for grinding, having a decreasing grain size in the P200-P1200 range, giving to the same surface of the paraboloid a “mirror-like” appearance, i.e. a mirror-like surface having a mean surface roughness (Ra) equal to 1/1000 with respect to the 1-mm wavelength (300 GHz). Instead, a 10-μm surface roughness (Ra) is obtained directly at the output from the process of metal deposition, without further finishing operations, it also with a value equal to 1/1000^(th) of 1 cm, referred to the 1-cm wavelength for 30 GHz frequencies. Thus, with the process it is possible to cover a very wide range of frequencies (up to 300 GHz). Therefore, the process described herein enables to obtain a reflective surface for an antenna with a resistivity in ohms near to zero in any point of the satellite dish. At the same time, the parabolic antenna undergoes no deformation, perfectly maintaining the parabolic pattern of the surface itself and with the features of lightness of weight and of sturdiness of the underlying carbon fiber structure, and with a suitable adhesiveness on all of the paraboloid surface.

The process of coating of the present invention can be applied to parabolic antennas of different diameter, preferably from 30 cm to 10 m, for instance with parabolic antennas made of carbon fiber reinforced epoxy resin (CFRP), of 1, 2, 3, 4, 5, 6, 7, 8 m. Preferably, surfaces of parabolic antennas made of carbon fiber (e.g., T300 3K fiber) in epoxy resin matrix, with a surface thickness comprised between 10 and 30 μm, will be used. The typologies of epoxy resins used are, e.g., of DT121R-47 and AX002-240-600F type, commercially available from DeltaPreg. The properties of the carbon fabric impregnated with epoxy resin are preferably in accordance with the directions of UNI and ASTM Standards. Preferably, antenna surfaces wherein the carbon fabric has an areal weight comprised between 190 and 210 g/m² (measured according to Standards UNI EN 12127) and a thickness comprised between 210 and 290 μm (measured according to Standards UNI EN ISO5084) could be used; in fact, the best performances are obtained with this type of antennas. Preferably, the carbon fibers of said antennas will have a tensile strength of at least 1700 MPa (measured by ASTM D-3039 testing).

Comparative experiments demonstrated that other processes of metal deposition do not afford reflective surfaces of antennas with the advantages had by the surfaces obtained.

Therefore, a further object of the present invention are reflective surfaces of parabolic antennas with a metal coating obtainable according to the above-described process and the parabolic antennas comprising them.

Parabolic antennas according to the present invention could advantageously be used for aboard communications at sea, radars, radiometers, radio telescopes and Earth observation satellites, as well as for other applications.

EXAMPLES

The present description will now be further detailed by the following examples, but in no way being limited thereto.

Example 1 Materials

The dishes used were constructed of CFRT (Carbon Fiber Reinforced Type) carbon fibers in epoxy resin matrix and with epoxy resin standard finish of 15-20 μm. The typologies of epoxy resins used were of DT121R-47 and AX002-240-600F type, commercially available from DeltaPreg. The properties of the resin-impregnated fabric are in accordance are in accordance with the directions of UNI and ASTM standards. The features of the epoxy resin weft and of the carbon fabric of this type of antennas are reported in the following Tables:

TABLE 1 resin matrix technical features Nature of the formulated product Thermosetting epoxy Cure temperature 110 ÷ 145° C. Gel time 8 ÷ 13 min at 120° C. 3 ÷ 6 min at 135° C. Tg [Cure cycle] 120 ÷ 125° C. [90 min at 120° C.] 120 ÷ 125° C. [40 min at 135° C.] Viscosity Medium-low Transparency Excellent Yellowing stability Excellent

TABLE 2 Carbon fabric features Yarn type (Warp; Weft)/ High-strength 3K Carbon Weaving style cloth Standard width/  1000 ± 5 mm Standard length    50 ± 5 m Warp (threads/cm)  4.90 ± 0.10 Weft (threads/cm)   198 ± 8 g/m² Dry fiber areal weight 0.260 mm (4) Resin content   47 ± 3% by weight Lamination thickness 0.260 mm Storage 4 weeks at 20° C.

Preparation and Deposition of the Metal Layer on the Reflective Surfaces of the Satellite Antenna

Material utilized for the surface coating: Aluminum wire; Process fluids: acetylene, oxygen and compressed air, and crystals of solid CO₂. Cooling: compressed air and compressed air which transfers solid CO₂ in granules/powder on the external surface of the satellite dish in rotation at 250 rev/min; Rotational speed under testing: 250 rev/min of satellite dish; Speed of coating robot: 22 mm/sec Distance from surface of satellite dish: constant in the range of 5-20 cm in order to avoid local overheating. Epoxy resin-impregnated carbon fiber external surface pretreatment: removal of 5-6 μm of resin. End roughness of pretreated surface (sandblasting with glass beads) about 3 μm. Shot peening of the surface layer of epoxy resin matrix was performed according to the following procedure:

The satellite dish of carbon fiber was positioned inside a closed-circuit soundproofed chamber, in order to avoid outside dispersion of glass microspheres. Steerable nozzles provide sending on the satellite dish surface a current of air in which glass microspheres having an average diameter of 1/10^(th) of mm are dispersed; said operation can also be carried out by a properly protected operator. A reference sample allows to check the removed thickness and measure the surface roughness Ra at the end of the process. The shot peening process consists in a surface hammering, performed cold by a violent jet of spherical glass beads, and, besides making the surface uniform and homogeneous, produces residual compression strains in the surface of the material and in the underlying layers thereof, which succeed to reduce internal strains. When the satellite dish is subjected to stresses during antenna use, the material is made more resistant to fatigue stresses.

Other parameters are:

-   -   Additional thermal shielding: with SbSn alloy (white metal)         wire, 100 μm thickness;     -   Aluminum coating thickness at the end of the coating process:         100 μm, without polishing;     -   Coating thickness at the end of the aluminum coating process:         350 μm, with final polishing;     -   End-of process roughness of the coating surface: 11 μm;     -   End-of-process roughness, with final manual polishing: 1 μm;     -   Adhesiveness of the aluminum film on the carbon fiber surface:         4-8 MPa;     -   Electrical resistivity, in ohms, on all points of the parabolic         surface: close to zero;     -   Possibility of coating satellite dishes of CFRP (carbon fiber         reinforced with epoxy resin) of any diameter, from 30 cm to 10 m         and above.

2. Performance-measuring tests conducted on antennas for satellite communications before and after the metallization process described in Example 1

The tests were conducted at a 30 GHz frequency on two identical samples of antenna, before and after the process described in Example 1; both samples were made of carbon fiber, with the reflective layer of T300 3K fiber (200 g/m²) as described in detail above. One of the two samples was coated with the metallization technique according to the embodiment of Example 1.

The graphs of FIGS. 1-4 show the pattern of the normalized gain of the antenna as a function of the angle on the horizontal plane (azimuth) and therefore provide the horizontal radiation pattern, over an angle comprised between −30 and 30 degrees with respect to the main axis of symmetry of the antenna.

Such diagrams provide a very important indication for checking antenna performance, both in terms of efficiency and of limiting emissions in undesired directions; in fact, satellite antennas for bidirectional communications, by being transmitting, are subject to strict limitations set by International bodies in order to limit spurious emissions toward adjacent satellites, therefore the radiation pattern is a fundamental element (though not the sole one) for checking the compliance with such regulations.

The measurements clearly show a decided improvement in the radiation pattern in favor of the metallized antenna, as evidence of the fact that the performances of the mere carbon fiber tend to decrease with the increasing of the frequency.

Such an antenna, in fact, was developed to work at markedly lower frequencies, in ku-band (10-14 GHz), where it provides sufficient performances. At 30 GHz the weft fabric of carbon fiber does not provide the required performances anymore, as it becomes partially transparent, and introduces signal diffraction phenomena which “foul” the radiation pattern with emission lobes in undesired directions. This is due to the fact that the wavelength becomes comparable with the dimensions of the weft of the fabric, which does not behave as an ideal surface anymore, by being not perfectly conductive. The metallization treatment described herein ensures a perfect electrical conductivity and an extremely accurate surface geometry, restoring the radiation pattern to its ideal shape, symmetrical and with extremely controlled and contained side lobes.

In the comparison between FIG. 2 and FIG. 4, in FIG. 2 it is evident the presence of 2 main lobes instead of only one, and the presence of relevant secondary lobes until above 5 degrees, whereas in FIG. 4 a first lobe is had at 0.8 degrees and a second lobe, already quite dampened, at 1.6 degrees, and then there are no further significant lobes. 

1. A process of coating the reflective surfaces of parabolic antennas, wherein said reflective surfaces are made of a composite material consisting in carbon fibers in epoxy resin matrix and wherein said epoxy resin matrix has a surface thickness comprised between 10 and 30 μm, comprising the following steps: a) performing a step of abrasion by shot peening on said reflective surface so as to remove a surface layer of epoxy resin of at least 3 μm; b) performing a step of metal deposition on the reflective surface prepared at a) by thermal wire spray technique with one or more phases of cooling on the acceleration current of the metal particles and/or on the surface of the parabolic dish of said antenna.
 2. The process according to claim 1, wherein said carbon fibers have an areal weight comprised between 190 and 210 g/m² and a thickness comprised between 210 and 290 μm.
 3. The process according to claim 1, wherein said step of metal deposition is performed by imparting at the same time a rotational speed to said antenna in the range of 40-400 rev/min.
 4. The process according to claim 1, wherein said cooling consists in blowing compressed air with crystals of solid CO₂.
 5. The process according to claim 1, wherein in said step of coating are used oxide-acetylenic nozzles or nozzles for oxygen mixed with combustible gases.
 6. The process according to claim 1, wherein said metal coating is of a metal or metal alloy selected from the group of Al, Zn, steel, Cu, SbSn Ag, Au, Ti, Cr, Ni molybdenum, Inconel, Monel, nickel/aluminum, bronze.
 7. The process according to claim 1, wherein the surface thickness of said epoxy resin is comprised between 15 and 20 μm.
 8. The process according to claim 1, comprising a further step c) of grinding said reflective surfaces.
 9. The process according to claim 1, wherein said carbon fibers are of T300 3K type.
 10. The process according to claim 1, wherein the removed surface layer of epoxy resin is comprised between 3 and 6 μm.
 11. The process according to claim 1, wherein the step of abrasion by shot peening is performed by glass microspheres of a diameter comprised between 1 and 10 mm.
 12. Reflective surface for parabolic antennas with a metal coating obtainable by the process of claim
 1. 13. Parabolic antenna comprising a reflective surface according to claim
 12. 